Much work has been done over the past few years in devising specific alloy compositions, either for superalloys with columnar grains, or else for superalloys of a monocrystalline structure.
In comparison with superalloys of similar composition and equiaxial grains, superalloys with columnar grains obtained by directed solidification have greater mechanical strength in respect of forces applied in the direction of the joint planes between the grains, but in respect of transverse forces their mechanical strength is no better.
The development of monocrystalline superalloys (in particular the alloy known as MAR-M200) was an important step forward in increasing strength in respect of forces applied transversally to the direction of solidification. But a comparable improvement could also be obtained in columnar grain superalloys by introducing hafnium in their composition, which for a time lead to much less work being done on monocrystalline solidification.
Interest in monocrystalline solidification has recently increased again, but for alloys of relatively simple composition, in particular without the addition of carbon, boron or zirconium. These elements were previously included in the composition both of monocrystalline alloys and of columnar grain alloys for the purpose of hardening the joints between the grains (at least in columnar grain alloys) to avoid premature breakage due to creep.
The said elements now avoided contribute to the formation of low melting point regions, particularly in the spaces between dendrites as they form during solidification. Avoiding said elements results in a notaable increase in the incipient melting point of the alloy, and thus makes it possible to consider using very high temperature heat treatment, particularly for nickel based alloys, in order to ensure that the .gamma.' (gamma prime) phase of the Ni.sub.3 (Al, Ti . . . ) type is completely in solution.
Alloys have been obtained in this way which can be heated to a high enough temperatue to ensure that the .gamma.' phase is completely in solution, and then to obtain controlled precipitation thereof up to a very large proportion, 60% or more, of the total volume.
Further, the size of the precipitated grains constitutes another significant factor governing the alloy's resistance to creep: for a nickel based superalloy having 60% or more of its volume in the form of .gamma.' precipitate, the optimum .gamma.' precipitate grain size for maximum resistance to creep is generally about 3000 .ANG. (.ang.ngstrom units). However, to obtain .gamma.' precipitate grains that do not exceed 3000 .ANG. in size, the precipitation temperature must be less than a fixed limit temperature.
One aim of the invention is to provide a composition for nickel based monocrystalline superalloys whose resistance to creep over a wide range of temperatures can be greatly increased by heat treatment.
Another aim of the invention is to provide a method of heat treatment which is particularly applicable to monocrystalline superalloys of such composition, to confer thereto an exceptionally high resistance to creep over a wide range of tempertures.